Now, without providing specifics about Hawkins' alleged activity, the SFO says its investigation has turned up nothing criminal and it is ending its involvement.

"Following a thorough evaluation where investigators considered thousands of documents, conducted interviews and obtained expert industry advice, the office has concluded that there are no grounds to commence a criminal prosecution," acting chief executive Simon McArley says.

The parties involved seemed to have "genuinely differing understandings of the contracts in question", and there was no evidence to prove Hawkins set out to deceive anyone, he says.

"While the manner in which the parties conducted their business appears to us to be unusual, we have evidence from industry experts that it is entirely possible that those involved honestly believe the conduct was legitimate.

"In light of that it cannot be concluded that the criminal threshold has been reached."

Mr McArley says while the investigation did not result in a prosecution it provided the SFO with a "deep insight into the contracting and sub-contracting nuances of the construction industry".

It is likely the SFO will look into these aspects of the construction industry again, Mr McArley says, with what is now a greater understanding of its "nuances".

Hawkins, which remained tight-lipped throughout the investigation, says it's glad the investigation is over.

"We have maintained all along that there has been no intent to defraud by Hawkins or its employees," general manager Dan Ashby says.

"It's been particularly difficult for our employees involved in the Hobson Street project, and clearly we are pleased it is over.

"Despite the distraction and stress the investigation has caused, we cooperated fully with the SFO and appreciate that they gave priority to the investigation, given the significant industry and public interest."